The fact that a significant fraction of Urban Solid Waste (U.S.W.) (paper, vegetables, food and wood) contributes in an important way to the Urban Solid Waste content of cellulose, potentially convertible into glucose by hydrolysis and then into ethanol by fermentation, has encouraged some countries to develop this route in order to create a new source of renewable energy (bioethanol), at the same time as reducing the growth of U.S.W. The use of the organic fraction of U.S.W. as raw material for the production of ethanol would allow the recovery of waste by the obtention of clean fuel with a great future.
A first solution for the ever greater accumulation of U.S.W. has been the utilization of that fraction with acceptable characteristics as fuel, in such a way that thermal and electrical energy are simultaneously produced (cogeneration), or exclusively electrical energy (generation) is produced. In this type of activities, profitability is not the sole factor to take into account.
The use of U.S.W. as raw material for the production of ethanol fuel can help not just to release the pressure caused by an increase in the surface of the dump, but also to provide energy in the most sustainable way possible. The facilities for transforming U.S.W. into ethanol would be able to provide a local solution to the accumulation of this waste, without requiring any thermal process (which generates great social opposition), and using instead of thermal processes a widely accepted biotechnological process of hydrolysis and fermentation. The organic fraction of U.S.W. does not have the problem of collection and transport raised by agricultural waste since it is systematically collected in well-established treatment plants. Moreover, these raw materials for ethanol production are not subject to seasonality, so that their availability and storage raise fewer problems than agricultural waste.
Raw materials for the production of renewable biofuels currently have their prices tied to markets that are not directly related to the automotive fuel market. The cost of sugar and cereals is regulated by food markets and their prices are high. The search for low-cost raw materials as alternatives to traditional ones is crucial for reducing costs in the production of bioethanol and for ensuring the profitability of projects. The use of lignocellulose biomass is, in the medium term, the most promising option for obtaining ethanol fuel at low cost.
The production of ethanol starting from U.S.W. would provide a revolutionary solution to various environmental problems since it generates a renewable fuel at the same time as eliminating waste.
There exist developments of some technologies for processing of cellulose found in U.S.W., though there are no industrial facilities in operation. In the last decade, intensive research has been conducted in this area, primarily in the United States, and various alternatives have been considered based on acid hydrolysis (Masada Resource in the USA) and enzymatic hydrolysis (Iogen in Canada). In Europe there do not exist any developments that could provide technologies that will produce a solution for all the stages that are required in a complete U.S.W. transformation process.
The production of ethanol starting from matter of agricultural origin has been a sufficiently developed technology at the industrial level since the end of World War Two. Nevertheless, these technologies have not been applied for treatment of U.S.W., specifically its cellulose fraction, no doubt due to the greater complexity entailed and the lower yields in ethanol production that are achieved.
Various patents have so far protected different variants of methods for the treatment of waste and the obtention of biofuels, among others:
U.S. Pat. No. 4,009,075 discloses a method for manufacturing alcohol—ethanol—starting from cellulose material coming from waste, by means of hydrolyzing that material into sugars and subjecting the resulting mixture to simultaneous digestion and fermentation in order to convert the sugar into alcohol and using an inoculum containing a cellulose enzyme and yeast. The method comprises the sterilization of the cellulose material, distillation in vacuo in order to isolate the alcohol and the recovery of the inoculum for its reuse. The sterilization of the cellulose material consists of a treatment with steam, sufficient for eliminating strains of bacteria that can cause undesired reactions.
ES-2166316 discloses a method for the production of ethanol starting from lignocellulose biomass, which comprises carrying out a pretreatment of the biomass consisting of grinding the biomass and subjecting it to steam explosion at a temperature of 190-230° C. for between 1 and 10 minutes, collecting the pretreated material in a cyclone, separating the liquid and solid fractions by means of filtration, treating the solid material in a fermenter, adding a cellulase and β-glucosidase and inoculating the fermenter with Kluveromyces marxianus. 
U.S. Pat. No. 3,990,944 discloses a method for obtaining ethanol by means of reactions under anaerobic conditions of a cellulose material, a cellulase and a microorganism that produces alcohol, the starting materials coming from agricultural crops such as rice straw, wheat straw, wood, cotton, corn leaves, even newspapers, corrugated board and bits of paper. The starting material is broken up and sterilized using heat and, simultaneously to the enzymatic hydrolysis by means of the addition of cellulase—or a producer microorganism of cellulase—the microorganism is added (for example, Saccharomyces cerevisiae) that produces the fermentation to alcohol.
U.S. Pat. No. 6,267,309 discloses a method for producing ethanol starting from urban solid waste which—according to one embodiment—comprises a pretreatment with dilute sulfuric acid in order to solubilize the remaining heavy metals and produce a soluble component and another insoluble one, separate the soluble component from the insoluble one, dry the insoluble component, treat the insoluble component with concentrated sulfuric acid in order to provide a partially hydrolyzed mixture, dilute said mixture, stir and separate the solids, concentrate the filtrate in order to obtain a part rich in sugar and to ferment the sugar. No simultaneous enzymatic hydrolysis and fermentation is carried out.
U.S. Pat. No. 5,407,817 discloses a method for treating solid waste that comprises its pretreatment with dilute sulfuric acid in order to reduce the heavy metal content, this method being similar to that disclosed by U.S. Pat. No. 6,267,309.
U.S. Pat. No. 4,321,328 discloses a method for producing ethanol starting from materials containing cellulose, which comprises saccharification, fermentation and distillation in order to recover ethanol, though in which the saccharification and fermentation are carried out separately.
EP-0005703 discloses a method for treating waste such as wastewater, domestic waste, etc., according to which the waste—after eliminating recyclable materials such as glass, etc.—is hydrolyzed by sulfurous anhydride—present in the water—in order to convert the hydrolyzable compounds into fermentable materials and finally into fuel such as ethanol. The saccharification and fermentation are carried out separately.
GB-1,604,948 discloses a method for simultaneously obtaining liquid fuel and a nutritive product starting from solid waste. Said method comprises subjecting the biomass to acid hydrolysis and converting it into a fermentable sugar. The acid hydrolysis is produced in the presence of sulfurous acid. No enzymatic hydrolysis is carried out.
U.S. Pat. No. 4,093,516 discloses a method for converting urban waste into liquid fuel, said method comprising partial concentration, saccharification, fermentation and distillation. The hydrolysis and saccharification are carried out simultaneously using chemicals and then—after deactivating the hydrolytic component and adjusting the pH and solid contents—the fermentation is carried out. No enzymatic hydrolysis is carried out simultaneously with the fermentation.
U.S. Pat. No. 4,237,226 discloses a method for treating cellulose substances prior to their hydrolysis with the aim of increasing the sugar yield and reducing the time for that hydrolysis. The hydrolysis can be chemical or enzymatic. This patent is therefore directed to a pretreatment of the cellulose material and does not disclose any later stage of fermentation and recovery of alcohol.
WO-9012103 discloses a method for producing alcohol starting from waste containing cellulose, and which comprises separating the materials that do not contain cellulose, hydrolyzing the cellulose materials using chemicals or enzymes, adjusting the pH in a storage tank, fermenting in a fermenter, eliminating solids and distilling the ethanol.
EP-1,690,944 discloses a method for obtaining alcohol starting from lignocellulose materials, which comprises chemical and/or physical pretreatment of a cellulose or lignocellulose substrate, enzymatic hydrolysis of the pretreated substrate and ethanolic fermentation by a microorganism.
Among the unpatented literature related to the present invention, the following publications can be cited:    Ballesteros, I., Oliva, J. M., Negro, M. J., Manzanares, P., Ballesteros, M. Applied Biochemistry and Biotechnology, 98-100, 717-732 (2002);    Ballesteros, I., Oliva, J. M., Negro, M. J., Manzanares, P., Ballesteros, M. Process Biochemistry, 38(2), 187-192 (2002);    Ballesteros, I., Oliva, J. M., Negro, M. J., Manzanares, P., Ballesteros, M. Grasas y Aceites, 53(3), 282-288 (2002);    Ballesteros, M., Oliva, J. M., Manzanares, P., Negro, M. J., Ballesteros, I. World Journal of Microbiology & Biotechnology, 8, 559-561 (2002);    Negro, M. J., Manzanares, P., Ballesteros, I., Oliva, J. M., Cabañas, A., Ballesteros, M. Applied Biochemistry and Biotechnology, 105-108, 87-100 (2003);    Oliva, J. M., Saez, F., Ballesteros, I., González, A., Negro, M. J., Manzanares, P., Ballesteros, M. Applied Biochemistry and Biotechnology, 105-108, 141-153 (2003);    Negro, M. J., Manzanares, P., Oliva, J. M., Ballesteros, I., Ballesteros, M. Biomass and Bioenergy, 25, 301-308 (2003);    Oliva, J. M., Ballesteros, I., Negro, M. J., Manzanares, P., Ballesteros, M. Process Biochemistry, 39, 1843-1848 (2004);    Oliva, J. M., Ballesteros, I., Negro, M. J., Manzanares, P., Ballesteros, M. Biotechnology Progress 20 (3), 715-720 (2004);    Oliva, J. M., Manzanares, P., Ballesteros, I., Negro, M. J., González, A., Ballesteros, M. Applied Biochemistry and Biotechnology, 121-124, 887-99 (2005).
The application of the new method for the treatment of urban waste will permit the recovery of a waste, reducing the harmful effects caused by the accumulation of urban solid waste in controlled dumps.